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Malaria and sickle cell trait

The blood cell abnormality sickle cell trait protects against malaria because of its effect on the immune response.

While it has been known for some time that sickle cell trait offers protection against malaria, the mechanisms have never been clear.

Now, a study - led by Dr Tom Williams of the Kenya Medical Research Institute/Wellcome Trust Research Programme in Kilifi, Kenya - has discovered an unexpected link with immunity. Children with sickle cell trait have shown enhanced immunity to the malaria parasite, with the level of immunity increasing with age.

Sickle cell trait occurs when someone inherits a normal haemoglobin gene from one parent (HbA) and a sickle haemoglobin gene (HbS) from the other (resulting in HbAS). While children with sickle cell trait do not usually display symptoms, children who inherit a double dose of the sickle gene suffer from sickle cell disease, which can cause chronic ill health and early death.

Further research will be needed to find out why the trait leads to increasing levels of immunity. One possibility, suggested by Dr Williams, is that a malaria infection persists in the body for longer, so allowing the immune system time to build up a better defence.

The blood cell abnormality, sickle cell trait, gives an increasing amount of protection against malaria as young children grow during their first ten years of life, new research has revealed.

Between the ages of two and ten immunity to the disease, which kills up to two million people a year, rises rapidly, a Wellcome Trust-funded study has found.

The project involved over 1000 people living on the coast of Kenya, where malaria is rife. Those taking part were aged from three months to 84 years, although most were under ten. The research, which is reported by the Public Library of Science today (31 May 2005), was carried out over six years, and compared the incidence of mild malaria in those with sickle cell trait and those
without.

The trait causes blood cells to deform – into the shape of a scythe – but does not cause sickle cell disease, which, in its worst form, can cause anaemia, lung problems and strokes. People with the trait, which is inherited, lead normal lives.

The study, which was led by Dr Tom Williams of the Kenya Medical Research Institute/Wellcome Trust Research Programme in Kilifi, found that the increased protection from sickle cell trait (HbAS) rose from 20 per cent in the first two years of life to over 50 per cent by the age of ten.

Dr Williams said: "It has been known for some time that sickle cell trait offers this protection, but the accelerated level of immunity in the first years of life has not been revealed before.

"There are several possible reasons why this happens and further research will be required before we find the full answer.

"But one explanation may be that HbAS causes the malaria infection to stay in the body a long time, so allowing the immune system time to build up a proper defence.

"Our research concentrated on mild malaria, which causes fever, but does not kill. We do not know if our findings can be applied to severe forms of malaria, which can lead to death."